The present invention relates to apparatus for measuring and magnetically recording events from two independent sources. In the electric power industry, such a device is sometimes referred to as a "totalizer" in that it sums or totals measured events from more than one source or meter. Such a measured event may be, for example, the consumption of units of real power (Kw) or reactive power (KVAR).
Electric power companies use magnetic recorders for billing and survey applications. Typically, an electric power meter of the conventional rotating disc type is equipped with a pulse initiator which generates an electrical pulse for each "event" which is detected or measured. Each pulse thus represents the measurement of a predetermined quantity of real or reactive power that has been consumed (or transmitted, in the case of two power company's sharing an intertie).
The pulses representative of the measured events are recorded on a magnetic recorder having a slow moving tape (for example, the tape may move at 1.75 in. every fifteen minutes). Along with the event or data pulses, a time mark is also recorded by the recorder. Such time marks typically record a fifteen minute interval, which may be a demand interval. These intervals may be used to apply different rates for power consumption.
When a meter of this type is read, the magnetic tape is removed from the recorder, and a new tape replaces it. The recorded tape is then taken to a remote location where it is read by a device referred to as a translator.
One common instance in which totalizers are used is when power companies have their power feed lines interconnected (called an "intertie") so that if a demand is made on one utility that is above its present operating capacity, power may be drawn from the second utility. Energy flowing between the utilities must be measured so that a record can be kept indicating what quantities of power were used by each utility, and the time periods during which such power was consumed. The electric meters used in such instances may be equipped with pulse initiators, as described above. For example, a pulse initiator may generate an electrical pulse for each revolution of a rotating disc on a conventional power meter.
For billing purposes, it is desirable to record these pulses on a magnetic recorder with associated time interval pulses, and there are a number of such magnetic recorders available for billing or for survey applications. The two kinds most commonly used in the industry are two-track and four-track magnetic recorders. One recording track is usually set aside for recording time pulses or marks which define the demand intervals which are typically fifteen minutes. Of the remaining three tracks on a four-track magnetic recorder two may be used for measuring real power, one for each power company, and the remaining track may be used for measuring reactive power for both companies.
One system for recording outputs from two separate meters on a single data track on a magnetic recorder is described in co-pending, co-owned application of Dyer and Hyde for TOTALIZER APPARATUS FOR RECORDING TWO DATA INPUTS ON A SINGLE CHANNEL, which was issued as U.S. Pat. No. 4,197,451 on Apr. 8, 1980. In this system, the totalizer generates a train of recording pulses at a predetermined "offset" repetition rate, preferably in the mid range of the channel capacity of the recorder. For example, if the data track of the recorder has a capacity of 6,000 pulses per hour, the offset repetition rate is chosen to be 3,000 pulses per hour. If neither input meter is recording power, the totalizer will record pulses on the common data track at a rate of 3,000 pulses per hour. The totalizer increases the repetition rate of the recording pulses above the offset rate in response to the increase in repetition rate of signals received from one of the sources; and it decreases the repetition rate of the recording pulses below the offset rate in response to an increase in the repetition rate of the signals received from the other source. Thus, this totalizer provides a bidirectional capacity in that it enables the recorder to record events from two sources on a single data track. However, a predetermined portion of channel capacity, (namely, 3,000 pulses per hour) is allocated to each meter. If one of the meters records at a rate greater than 3,000 pulses per hour, all pulses over that rate will be lost, even though the recording capacity of the recorder is 6,000 pulses per hour.
The present invention is thus directed to a totalizer which is capable of recording the output signals of two separate meters on a single data channel, and in which the full recording capacity of the data channel may be used for either meter output if such capacity is available--that is, to the extent the other meter is not measuring events. To accomplish this, first and second first-in, first-out (FIFO) memories are used alternately in successive demand intervals to sum the outputs from both meters. In other words, during a first demand interval, the first FIFO memory records output signals representative of measured events from both meters, and during the next successive demand interval, the second FIFO memory records signals representative of events occurring in the next successive demand interval.
For recording, the contents of the two FIFO memories are shifted out in alternate demand intervals. The demand interval signals which are recorded on the time track of the recorder are delayed so as to be recorded in synchronism with the data pulses associated with that demand interval. Thus, the accuracy of recording is within one event or data pulse per demand interval. Each FIFO memory acts as a summing register for both meters. Thus, the proportion of available storage allocated to each meter depends upon the respective activity levels of the meters up to the maximum capacity of the recorder. If, for example, one of the meters is recording at a rate of 1500 data pulses per hour for a given demand interval, and the other meter records at a rate of 4,000 data pulses per hour for the same interval, the system will record at a rate of 5500 data pulses for the two meters for that particular demand interval, and the data within that interval will be accurate to within one pulse. This is the accepted accuracy of precision recorders.
Once a demand interval is terminated, the output pulses from the meters are recorded in the other FIFO memory, while recording on the data track from the first FIFO memory continues until that data is exhausted. Thereafter, when the time mark is recorded on the time recording track, the contents of the second FIFO memory are shifted to the output for recording on the data track.
Since the time mark pulse is delayed approximately only two seconds so that the exact number of data pulses are recorded in a given demand interval, the FIFO memories are not required to have, and in fact do not have, a large capacity. Rather, sixteen bit memories are sufficient for the desired operation.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing.